Audio content has one major advantage over video or written content: you can fully consume it while being engaged in another activity. Of course you can, say, also read a magazine while watching something on TV, but in that case it will be impossible to grasp the full details of both sources of content. Another example where watching video or reading is clearly not possible is in the car – if you are in the driver seat in this unfortunately still pre-Google-self-driving-car-for-commercial-use era. Listening to audio on the other hand is perfectly possible when you are behind the steering wheel.